Natural gas is generally sold by volumetric measurement, and one way to measure the flowing gas is by passing it through an orifice of a fixed, predetermined size and measuring the pressure drop across the orifice. An orifice meter or fitting comprises a housing within which a disc with an orifice is supported in a flow passage. The orifice fitting is installed in a gas pipeline, and is fitted with pressure taps for measuring pressure immediately upstream and immediately downstream of the orifice. Because the orifice plate is subject to wear by impacting of sand, line scale and other foreign particles in the flowing stream, it must be replaced at frequent intervals to ensure accuracy in measurement.
Existing orifice fittings are generally of two types. The most basic, or junior type, is simply a housing which is installed in the pipeline and includes a closure plate or the like to be removed for changing orifice plates. With this type, the flow through the pipeline section must be cut off or by-passed during the time that the housing is open for removal and replacement of the orifice disc. In the more sophisticated or senior orifice fitting, the valve body is provided with a separate chamber radially displaced from the pipeline, into which the orifice carrier may be moved while the gas continues to flow through the pipeline itself. However, even with such senior fittings, it is necessary to move the orifice disc carrier into the displaced chamber; to seal the chamber off from the pipeline itself; and to bleed the chamber before opening it for access to the orifice carrier for replacement of the orifice disc. The operation also generally includes the removal and replacement of a plurality of bolts or screws which secure the orifice disc onto the orifice carrier. This entire operation may require a considerable amount of time during which, customarily, the immediately preceding rate of flow is assumed to continue. However, this assumption may not be correct for the entire period of downtime, and the longer that period continues, the greater the chance for error.